The new banking regulator is investigating the IT meltdown at Royal Bank of Scotland that left millions of people locked out of their accounts or awaiting payments over several weeks in the summer of 2012.

The Financial Conduct Authority (FCA), which took over regulation of financial services companies at the start of April, said it had begun an enforcement investigation into the breakdown, which affected accounts at RBS, NatWest and Ulster bank; the latter two are alos part of the RBS group.

The authority said it would "reach its conclusions in due course and will decide whether or not enforcement action should follow that investigation". If it does find that there were systemic failures behind the technology problems, the bank could face a fine, or individuals could be censured and banned.

The technology meltdown at the state-backed bank began in mid-June when customers found they were unable to access their accounts online, or to withdraw or transfer money. Millions of RBS and NatWest customers were affected for several days, while accounts at Ulster bank did not operate as normal for many weeks.

As well as those who bank with RBS, the breakdown hit customers of other banks who were expecting salary payments or other transfers from accounts held at the bank. Since the summer, there have been other, less extensive problems with RBS systems, most recently in March when mobile banking customers were hit by a glitch in the bank's app.

The regulator does not usually announce enforcement investigations until they are finished, but is allowed to do so where there are exceptional circumstances. The FCA's predecessor, the Financial Services Authority has fined companies for IT failures, after finding that they had broken its rules.

A spokesperson for RBS said the bank would be working closely with the regulators in the UK and Republic of Ireland. "Last summer's IT failure was unacceptable. We have already made significant improvements and, over the next three years, will invest hundreds of millions in our systems," the bank said. "Our customers deserve a service they can rely on 100% of the time, and that's what we want to provide."